1. Field of the Invention
This invention relates generally to computer digital storage systems, and in particular to computer systems using a plurality of disk drives.
2. Description of the Related Technology
Personal computers have gained substantial popularity among individual users for both business and home use. Personal computers are now being utilized for jobs heretofore performed by mainframe computers and minicomputers. The rapidly growing popularity in the use of personal computers may, in part, be attributed to the substantial improvement in both its speed of operation and random access memory (RAM) capacity.
Presently, microcomputer processors may operate at 33 Mhz clock rates and utilize 32 bit data and address buses to access two to 256 megabytes of RAM. In general, RAM speed has kept pace with processor operational speeds, however, bulk data storage utilizing magnetic disks have not. To maintain computer system performance, larger main memories are being used to buffer frequent disk access requirements for data retrieval. This may be a solution for applications that will fit entirely in the main memory of the computer and for which memory volatility is not important. However, applications such as transaction-processing which have a high rate of random requests for small amounts of data or large simulations requiring massive amounts of data that are in excess of the main memory capacity require memory storage capacity only available by using disk storage technology.
Disk storage technology relies on the performance obtainable from mechanical devices. A disk storage device is comprised of a least one magnetic oxide coated platter and at least one read and write head. An electric motor is connected to a spindle that causes the platter to rotate at approximately 3600 RPM. The read/write head "floats" just above the surface of the disk platter oxide coating and moves back and forth across the face of the platter perpendicular to its rotational axis. The head moves predefined incremental distances called tracks. The disk platter is subdivided up into a number of tracks that form concentric imaginary circles on the platter face. Each track consists of a number of sectors that further divides the track into contiguously joined arcs forming a 360 degree circle. These sectors pass under the head as the disk platter rotates. A sector contains multiple bytes of data. A byte of data consists of eight bits binary information.
Data is stored as multiples of eight bit bytes in a disk sector. For example, a sector may contain 512 bytes of data, however, the sector may also contain more or less bytes depending on the disk system and its application. In those systems that utilize sectors of 512 bytes, if less than 512 bytes of data need be stored, then blank or dummy data is added to make up a full sector of data. If more than 512 bytes of data are stored then additional sectors are used.
Storage capacity of disk systems vary widely depending on platter size, number of platters, type of track and sector formatting and the precision of head position in mechanisms. Mass production of disk systems for personal computers have created low cost and high performance disks having data storage capacities of a hundred megabytes or more. Larger capacity disk systems, utilized in minicomputer and large mainframe computer systems, are typically several thousand megabytes or more. The cost per megabyte of a thousand megabyte disk is more than twice the cost per megabyte of the mass produced disks used in personal computers. Reliability is equivalent between either individual type of disk. However, power consumption and size are much lower per megabyte for the small disk systems.
In choosing a disk system for computer applications requiring thousands of megabytes of data storage one must evaluate two different approaches for implementation of a suitable disk system. The first and traditional approach utilizes one or more large capacity (thousands of megabytes) disks, the second utilizes a large number of smaller capacity disks. Redundant Arrays of Inexpensive Disks ("RAID"), based on magnetic disk technology developed for personal computers, offers an attractive alternative to the large capacity disks. RAID arrays offer improved performance, lower power consumption and lower incremental costs for additional capacity than do the large capacity disk systems.
A disk normally rotates at a speed of 3600 RPM. At this rotational speed the head passes over a particular sector every 16.67 milliseconds. Thus, the longest time required to align the head with the sector of interest would be less than or equal to 16.67 milliseconds. This delay is called rotational latency time. Data storage and retrieval latency time may be decreased by interleaving the read and write operations of a multiple disk system. Interleaving means that data is read from or written to alternate disks.
When interleaving data operations between multiple disks a first block of data is written to or read from disk A, a second block of data is handled by disk B, a third block of data is handled by disk C, a fourth block of data is handled by disk D, etc. After all data disks are so utilized, the interleaving cycle starts anew with the first disk A. Interleaving reduces the time required to transfer blocks of data because disk operations may be performed during the rotational latency times of the multiple disk system. For example, to write 10,240 bytes of data onto two disks, each disk having 512 byte sectors, would require the use of 10 sectors from each disk. The first block of data containing 512 bytes is written to or read from the first disk. The next block of data is written to or read from the second disk and subsequent blocks of data containing 512 bytes each are alternately transferred to/from each disk.
When data blocks in excess of 512 bytes are to be written to the disks, the disk controller may buffer an amount of data limited only by the amount of buffer or cache memory available in the controller. The disk controller writes this buffered data to the appropriate sectors of each disk. The controller must wait for the correct sectors to align with the write head before data can be transferred. When transferring data to a disk, a latency time of one revolution of the disk platter may result if the sector to be read or written has just passed the read/write head.
When using multiple disks in a disk storage system, synchronization of rotational speed and sector position may be accomplished by phase-lockedloop control which is well known in the art of disk systems. Phase-lockedloop control of the rotational speed and position allows rotational alignment for each of the corresponding sectors of the drives. Thus, sector 5 of disk 1 may be read or written at the same time as sector 5 of disk 2. Disk synchronization averages rotational latency to one half a revolution or 8.33 milliseconds instead of the possibility of a full revolution latency of 16.67 milliseconds.
However, even with interleaving of multiple disk operations and rotational synchronization, the disk system data transfer rates are abysmal compared to the computer system I/O bus data transfer capabilities. Disk controller systems have tried to increase the throughput rates of data transfers between the disks and RAM by utilizing cache RAM integral with the multi-disk controller to buffer blocks of data. The multi-disk controller reads the data in 16 bit words from the disks and may store this data in either 16 or 32 bit word buffer memory.
Current multi-disk controller technology utilizes 32 bit word cache memory buffers for data transfer to the computer system I/O data bus, however, the multi-disk controller processor logic only stores one 16 bit data word per read and write operation. In other words, the controller processor only loads 16 bits of data at a time from the disks and must use two read and write operations to load 32 bits of data into the cache memory buffer. The first read/write operation addresses the low 16 data bits of the cache memory address and the second read/write operation completes the high 16 bits of the 32 bit cache memory. Requiring two disk controller read/write operations per 32 bit data word may reduce in half potential data throughput. In transferring large blocks of data to the computer system RAM, this lost data throughput potential can be significant.